I had a hard time picking where to throw this one but I think a racing section is appropriate.

My question involves, as you could probably infer, sunroofs and their effect on the structural stability and handling of a car. I wanted to ask you all your feelings on it.

Either from personal experience or from techinical studying, how much do you feel the inclusion of a sunroom effects the stability of a car? Does the big hole in the roof let the body sway any more than without? If so, is it enough to contend with the positive handling advantages of strut/tower brace bars?

 

at a minimum it raises your center of gravity as it adds weight to the top of the car. now how much and what effects that has, I dont know.

 

hmm well how much more per square inch does glass weigh than body material?

 

Not sure, but the rails and motors add weight too. I think the sunroof adds ~35lbs when comparing Mitsu specs. As for rigidness/stiffness, I don't know.

 

add the fact that the weight is so far above the center of gravity. making it's weight all the more potent. I wish I could take measurements on this kind of thing. Any vendors that have any experience with this would be welcomed into adding some thoughts.

 

As far as rigidity goes, there's not a real difference. Yes, you now have a big hole in your roof. However, most manufacturers add more bracing around the hole so it pretty much balances out. I don't think you'd really notice any difference in stiffness bewteen a sunroof and non-sunrood model.

Also, it's true a sunroof adds significant weight to the very top of the car (where you want it least). Anywhere from 25-50 pounds depending on whether it's powered or not. But you probably wouldn't notice the difference on a 3200+ pound Evo unless you're really dialed-in to the feel of your car. Use this as your reference: the amount of gas in your tank makes a much more noticable difference in handling. If you're the kind of driver that can't tell if your tank is full or half-empty based on how the car feels, you can bet you wouldn't notice the extra weight associated with the sunroof!

The main disadvantage of a sunroof for me is the loss of headroom. If you use a proper driving position with a relatively upright seat and you're over 6-feet tall (as I am), then your head starts to get pretty close to the roof. Put on a helmet and your head is practically touching the headliner. Now lose 2 inches of headroom and try to get comfortable. Impossible.

Emre

 

It's not simply the weight of a sunroof, it's the effect on torsional rigidity that's the real problem. Modern cars are designed as stressed skin monocoques - that means that as far as possible, all loadpaths are designed to put the outer skin in tension. If you imagine putting the roof skin in tension you can see how detrimental a rectangular hole will be - it will lozenge immediately.

The roof is a critical section of a modern bodyshell, if the bodyshell was engineered with the assumption of a structural roof/pillars in place, never ever cut a hole in it, and if a hole absolutely has to be there, the worst possible shape for that hole will be a rectangle.

Charles

 

I agree complety with what ACM is saying in spirit. I have found, however, that having a sunroof hasn't been a problem when tracking my Evo. The car seems to be rigid enough that there isn't any noticeable deformation. Even after 7 or 8 hard track days, I have yet to hear any change in (or noticeable) sunroof wind noise when the car is being driven hard.

Kayaalp's comments about head room are dead on. I would bet that anyone over 6' would have a hard time with a sunroof and helmet. I'm 6' and have just enought room with nada to spare.

From what I've seen the driver's skill will make more of a difference than the sunroof.

 

I've noticed that the glass on the sunroof squeaks on the rubber seals when putting the car at funky angles going over certain bumps and depressions. I'm pretty sure it's the body flexing. Going over dirt roads at speed I do not notice that kind of squeaking, nor does it squeak during normal driving. I'm not a racer though, so I enjoy the sunroof's other benefits =)

 

What he said plus often to put the sunroof in structural members need to be moved from their ideal location or made in a less than ideal shape.

Having said that, for most people driver skill makes more of a difference. However, cars with sunroofs often show more structural weakness as they get older, eg. squeaks and groans and sunroofs that don't work so well.

 

A very technical explination, but I really dont know where your getting your information. If the body was that stressed, torsion points like the front strut towers would cause the fenders to flex, which does Not happen. I also question (read question, not disagree) why you say the worst shape would be a rectangle.

My arguement is based on these perspectives, and my 4 years of engineering at Purdue. First off, open the top of a soda can. (just the pop top) Can didnt lose much rigidity, did it? Also, look at how strong hollow pipe can be. This has to do with shear stress and moments of inertia, both being dependant on area.

Dont take this as an attack, Im just trying to have a conversation here. Im always up for more learning......

 

From a purist's point of view you don't want a sunroof. Additional weight in all the wrong places with compromised structure and reduced head room. My previous car, Porsche 993, had a sunroof. I never used it and cursed it every time I went to the track as I had to slouch/hunch considerably to fit with a helmet on -- and I am only 6'.

I have no such problems with my Evo as I ordered it without.

If you want to be able to see the sky, buy a roadster!

Elise anyone?

 

well I'm only 5'7" ... so I don't have quite the problems with height you do.

 

I am not an engineer but have some experience with race car chassis development.

Charles may have an additional response but the comparison with the front strut towers is problematic in a sense that there is sually a lot of heavy gauge sheet metal in that area that is also triangulated with multiple pieces of overlapping sheetmetal. Nevertheless strut tower bars are sometimes a good idea if you put enough force into the chassis.

The roof gives rigidity to an area that is usually the weakest link in a 4 door sedan due to the fact that there are 2 huge openings on each side that can't be triangulated unless you either weld them shut or put a cage in or both.

The comparison with the pop can analogy is problematic in that the rigidity comes from the circular reinforcement of the can where top and sides are fused. The pop can opening is triangular with rounded corners. Also if you put enough pressure on the top of the can you can usuallly tear open the edges of the opening. Try that on the bottom of the can where there is no opening.

 

Valid arguements, but heres my issues. Yes the strut towers are heavily braced components, but I was trying to disprove the concept of modern cars having "stressed skin monocoques." If that were the case, the strut towers would rely on the fenders for support, which they clearly dont.

Second, you negate the soda can concept in saying its the weld points on the can that give it strength. On the same lineage of thinking, its not the middle of the roof that gives it its strength, but rather the ends of the roof that have different pieces welded/formed together. If the center of the roof was what mattered most in structural rigidity, it would not be made of as thin of metal as it is. If you have time, look at the uniframe of the car, comparing roof thickness with uniframe thickness around the door / roof edge area.

You are correctly right about the two doors on each side being the weakest link in a sedan. This is why door jams have so much more material joined to them.

(Im trying to look for a stress analysis run using Pro-E for some car, it will show how little stress is seen in the middle of the roof during bouts with torsion.)